Valves used, for example, for controlling the flow of liquids in pipelines, eg in petrochemical plants, commonly include a valve stem which passes into the valve body. The valve stem must be suitably sealed at its point of entry into the valve body, in order to prevent leakage of the liquid. The seal is usually formed by a packing material arranged around the valve stem and compressed by a sleeve known as a gland to make a tight joint. The pressure exerted by the gland keeps the packing tightly pressed against the valve stem.
In valves of this type, it is important that the pressure exerted by the gland on the valve packing is neither too great nor too small. If the pressure is too great, the valve may be difficult to operate; if the pressure is too small, leakage may occur. For this reason, it is common to use calibrated spring assemblies to act on the gland. The optimum compressive force depends inter alia on the nature of the packing material and the various dimensions of the valve components, and can be calculated. A spring assembly producing this optimum force may then be prepared and utilised.
Such spring assemblies commonly comprise a stack of annular disc springs which are Moused in a cup shaped housing. The base of the housing has a central aperture which enables the assembly to be positioned about a screw-threaded stud, one end of which is fixed in the valve body and which passes through an aperture in the gland. A support washer is then placed on top of the spring assembly and tightened with a nut. Once tightened, the stack of springs acts between the support washer and the base of the housing, exerting the required force on the gland.
A disadvantage of spring assemblies of the type described is their loose assembly. This means that the integrity of the disc spring stack is easily lost prior to installation or during maintenance work. For the correct force to be exerted, it is necessary that the correct number of disc springs be used, and also the correct sequence. If the number and/or sequence of disc springs is incorrect, insufficient force may be exerted on the packing, leading to the danger of potentially hazardous leaks. Another disadvantage is that the support washer may be over-tightened, leading to excessive compressive force on the packing and difficulties in valve operation.
There has now been devised an improved form of spring assembly for use in a valve which overcomes or substantially mitigates the above mentioned disadvantages.